There are three main methods for measuring the optical properties of a liquid crystal, such as TN LCD, STN LCD and TN TFT LCD, in the prior arts. (1) The LCD to be detected is mounted between two polarizers each having an absorption-axis angle of 45°, and then a maximum absorption wavelength is obtained by a spectrophotometer or the device like that. Then, the phase shifting value (Δnd) of the liquid crystal display could be obtained from a predetermined equation relating to the maximum absorption wavelength and the Δnd, where the maximum absorption wavelength has a positive correlation with the Δnd. There are some disadvantages in this mentioned method. (a) It is unable to obtain the twist angle of the liquid crystal, i.e. the twist angle is necessary to be a known parameter. (b) It is unable to obtain the respective Δnds for various wavelengths. (c) Since the manufacturing process and the material of the LCD are the determinants of the correlation between the maximum absorption value and the Δnd, it is necessary to find out the respective correlations therebetween. (2) Based on the various twist angles of the liquid crystal, the LCD to be detected is mounted between two polarizers having the specific absorption angles, and then a maximum absorption wavelength is obtained by a spectrophotometer or the device like that. After that, the Δnd of the liquid crystal display could be obtained from a predetermined equation relating to the maximum absorption wavelength and the phase shifting value, where the maximum absorption wavelength has a positive correlation with the Δnd. There are some disadvantages in this mentioned method. (a) It is unable to obtain the twist angle of the liquid crystal i.e. the twist angle is necessary to be a known parameter. (b) It is unable to obtain the respective Δnds for various wavelengths. (c) Since the Δnd is obtained from the predetermined equation, there always exists an error therein. (3) The LCD to be tested is mounted between two polarizers, and a spectrophotometer or the device like that is applied. The absorption angles of the two polarizers are modified by try and error until the transmittance of the maximum absorption wavelength is the closest to zero and the maximum absorption wavelength would not change while rotating the panel. After that, it is able to obtain the twist angle of the crystal liquid, the Δnds after calculations, and then the correlation between the Δnds and the various wavelengths. The disadvantage for this method is the time waste during the try and error process.
As mentioned above, a device and a method for measuring an optical parameter of a liquid crystal display without the disadvantages of the prior arts are desires.